This invention relates generally to power control circuitry for an electronic ballast suitable for igniting a ceramic metal halide lamp and, more particularly, to an electronic ballast that utilizes power control circuitry to provide substantially constant power to a ceramic metal halide lamp regardless of the resistance of the lamp.
Ceramic metal halide (CMH) lamps provide several advantages over conventional metal halide lamps including a more uniform color spectrum. A CMH lamp includes a discharge tube defining an interior region. Upon application of sufficient voltage across a pair of electrodes positioned within the interior region of the discharge tube, a high pressure arc is ignited in the discharge tube interior region.
It is highly desirable that the power input to a CMH lamp operating under steady state conditions be maintained at a substantially constant value because the color of light emitted by the lamp is dependent on the power input to the lamp. As a CMH lamp heats up during operation, the resistance of the lamp decreases. Assuming a constant source voltage, the current through the lamp increases with decreasing lamp resistance. Thus, power input to the lamp would increase as the lamp heats up and the color of the light emitted by the lamp would undesirably shift or change.
Control of power input to CMH lamps is also important because CMH lamps have different resistance values due to variations inherent in the manufacturing process. Additionally, as a CMH lamp ages its resistance value increases. Thus, by accurately controlling power input to a CMH lamp is possible to avoid color shifting during operation and to have uniformity of emitted light color when a number of CMH lamps are used to illuminate an room or other area.
The present invention is directed to power regulation circuitry for an electronic ballast for igniting and maintaining an arc within a discharge tube of a ceramic metal halide (CMH) lamp. In a first embodiment of the present invention, the electronic ballast includes an inverter circuit for providing power to a CMH lamp and power regulation circuitry coupled to the inverter circuit to maintain substantially constant power to the lamp in spite of changes in the lamp resistance during operation.
The power regulation circuitry includes an operational amplifier configured as an integrating error amplifier, a pair of p channel MOSFET control transistors, a secondary winding coupled between the control transistors, a low pass filter, a integrating capacitor coupled in a feedback loop of the operational amplifier, and a sensor resistor coupled in series with one of a pair of complementary MOSFET transistors of the inverter circuit.
As the resistance of the lamp changes, the current through the sensor resistor and the voltage across the sensor resistor both change. The voltage across the sensor resistor is coupled to one input terminal (inverting input terminal) of the operational amplifier. A set point voltage is input to the other terminal (noninverting input terminal) of the operational amplifier. A set point voltage is selected such that a desired power is applied to the lamp. The set point voltage is derived from a reference voltage taken from the inverter circuit. The reference voltage is attenuated by an attenuator or divider circuit to generate the desired set point voltage.
The output of the operational amplifier is proportional to the voltage difference between two input terminals and is coupled to respective gate terminals of complementary MOSFET control transistors. A secondary winding is coupled between the drain terminals of the MOSFET control transistors. The secondary winding is inductively coupled to a primary winding in the inverter circuit. If the resistance of the lamp changes, for example, decreases as the lamp heats up, the current though the lamp and the sensor resistor will increase and the magnitude of the voltage difference the input terminals of the operational amplifier will increase.
If the voltage difference between the operational amplifier input terminals is of sufficient magnitude, the MOSFET control transistors will be turned on, shunting current away from the secondary winding and by inductive coupling reducing current through the inverter circuit primary winding. Reducing current through the inverter circuit primary winding increases the switching frequency of the inverter circuit. The inverter circuit, in turn, excites or drives an LC tank network coupled to the lamp. Increasing the frequency of the inverter circuit drives the LC tank network at a frequency further above the LC tank network resonant frequency and thereby decreases the voltage applied across the lamp by the LC tank network. Thus, as lamp current increase, lamp voltage decreases such that lamp power remains substantially the same.
In a second embodiment of the present invention, the power regulation circuitry further includes an inverting operational amplifier to insure that the set point voltage is maintained at the desired magnitude even if the DC bus voltage fluctuates because of fluctuations in the AC power line voltage. Without the inverting operational amplifier, changes in the DC bus voltage will cause corresponding changes in the reference voltage which, in turn, will cause the set point voltage to change. Recall that the set point voltage determines the power applied to the lamp. The output voltage of the inverting operational amplifier is used as the set point input voltage coupled to the noninverting input of the integrating error amplifier. The remainder of the power regulation circuitry is identical to the first embodiment.
These and other objects, advantages, and features of an exemplary embodiment of the present invention are described in detail in conjunction with the accompanying drawings.